Our long-term objective is a physical-chemical and molecular understanding of the processes of phototransduction in photoreceptor cells. The details of the sequence of events that mediates excitation by linking the absorption of photons by photopigments to changes of permeability of the plasma membrane are still poorly understood. Moreover, the sequence of events that modulate transduction processes to produce "adaptation" are also poorly understood. Our specific aims include the design and performance of experiments to test current hypothesis for intracellular signaling of excitation and adaptation in both vertebrate and invertebrate photoreceptors. We propose to compare the reversal voltages of membrane currents induced by injection of both Ca++ and cGMP into vertebrate rods with the reversal voltage of light-induced current, as measured in voltage-clamp. We also will examine the ability of Ca++ to control conductance channels in the plasma membrane of rod outer segments using the "patch-clamp" technique. We propose to re-examine the kinetics of the light-induced decrease of cGMP in rods by use of rapid microwave inactivation of rod enzymes. In addition, we propose to continue the examination of the kinetics and spatial distribution of light-induced changes of intracellular Ca++ in Limulus ventral photoreceptor cells using a rapid-scanning microphotometer. Also, we propose to investigate changes of voltage across intracellular membranes using voltage-sensitive dyes.